The present invention relates to fuel core loading for refueling a boiling water nuclear reactor (BWR). In particular, the invention relates to core loading strategies for the removal of expired (“burnt”) fuel bundles, installing fresh fuel bundles and shuffling existing fuel bundles to be reused in the core during a subsequent fuel cycle.
The core of a BWR comprises an array of fuel bundles. Each fuel bundle houses an array fuel rods formed of radioactive material. The bundles are arranged in the core based on design constraints such as radioactivity exposure limits for each bundle. The radioactivity of the core heats the water in the reactor. The reactor typically operates on a fuel cycle having a cycle period of, for example, a (1) year, year and a half (1.5) or two (2) years. At the end of each fuel cycle the reactor is shut down to refuel, maintain and repair on the core and reactor vessel. During refueling, fuel bundles are removed from the core, bundles to be reused are left in their current core location or shuffled to a new core for the next cycle, and fresh fuel bundles are installed in the core.
Cranes over the reactor vessel move fuel bundles during the fuel loading operation. To move an existing bundle, the crane is positioned over the bundle; captures the bundle; elevates the bundle out of the core; moves the core to a pool or to a new core location, and lowers the bundle to the pool or new core location. Several minutes are typically needed to move each fuel bundle by crane. During a single core loading process, some of the bundles are moved twice, e.g., from a core location to a pool and back to a new core location. In the past, core loading plans and maps have been complex and involved moving most fuel bundles in a core.
There are several hundred bundles in a core. To move all or just most of the bundles can require many days. The fuel loading process has often required seven to fourteen days to complete, and rarely has been completed in less than three days. Although bundle placement mistakes are rare, the risk of placing a bundle at an incorrect core position increases with the number of bundles to be placed. There is a long felt need to reduce the number of bundles to repositioned, e.g., shuffled, in a core.
Fuel bundles are arranged in a core pursuant to a core loading strategy. A workable core loading strategy typically ensures that the core and fuel bundles adhere to thermal margins (e.g., heating in critical power ratio (CPR) and along the length of individual bundles (kw/ft)) and reactivity margins (e.g., hot excess (HOTEX), shut down margin (SDM), and end of cycle (EOC) energy)). Shuffling fuel bundles during refueling is often used to meet thermal and reactivity margins for the core and individual bundles. The core loading strategy may also ensure that fuel bundles do not exceed the excessive limits. The core loading strategy also determines the location of location and type of fresh fuel bundles to be loaded during each refueling operation.
It has not been uncommon for a conventional core loading strategy to shuffle most or all of the exposed fuel bundles to be reused. Extensive bundle shuffling was done to provide appropriate thermal and reactivity margin limits. Excessive shuffling increases the time needed to refuel a core and increase the risk that bundles will be placed in an incorrect core location.
A core strategy may have as a design target an “equilibrium” core that adheres to the thermal and reactivity margins and other core design criteria. An equilibrium core has minimal changes in the loading of its core fuel and the exposures of bundles in the core between successive fuel cycles. An equilibrium strategy promotes use of the same fuel bundle loading plan, fresh fuel definitions and rod pattern depletions from cycle to cycle. An equilibrium approach to core loading minimizes the core locations: from which burnt bundles are to be removed, fresh bundles are to be inserted and from which bundles to be shuffled are taken and placed does not significantly change from one core loading cycle to another. Equilibrium typically requires several fuel loading cycles, e.g., 8 to 10 cycles, to achieve. The equilibrium core loading plan represents a desired target to be achieved in a core loading strategy that extends over many fuel cycles. The “equilibrium” strategy assists vendors and customers to develop a long-term core loading strategy for economic and scheduling considerations. An equilibrium core loading plan may be used to compare one core loading strategy to another.
To reduce the time needed to load fuel bundles, a method and system are needed for reduce the fuel bundles to be moved during each refueling operation. A method and system for fuel loading should take into account a core loading strategy for fresh fuel bundles, removal of burnt fuel bundles and reuse of bundles during two or three successive fuel cycles. Selecting burnt bundles to be removed, identifying existing bundles to be shuffled and determining their new core locations, and selecting fresh bundles and identifying their core locations are determined by a core loading strategy.
There is a long felt need for a core loading strategy that simplifies the core loading process and reduces the time needed to remove, shuffle and load fuel bundles in a core. There is also a long felt need for a core loading strategy that achieves equilibrium in a reduced number of loading cycles and an equilibrium having a small change in core loading between successive loading cycles.